Time course of ERP generators to syllables in infants: a source localization study using age-appropriate brain templates.

Event-related potentials (ERPs) have become an important tool in the quest to understand how infants process perceptual information. Identification of the activation loci of the ERP generators is a technique that provides an opportunity to explore the neural substrates that underlie auditory processing. Nevertheless, as infant brain templates from healthy, non-clinical samples have not been available, the majority of source localization studies in infants have used non-realistic head models, or brain templates derived from older children or adults. Given the dramatic structural changes seen across infancy, all of which profoundly affect the electrical fields measured with EEG, it is important to use individual MRIs or age-appropriate brain templates and parameters to explore the localization and time course of auditory ERP sources. In this study 6-month-old infants were presented with a passive oddball paradigm using consonant-vowel (CV) syllables that differed in voice onset time. Dense-array EEG/ERPs were collected while the infants were awake and alert. In addition, MRIs were acquired during natural non-sedated sleep for a subset of the sample. Discrete dipole and distributed source models were mapped onto individual and averaged infant MRIs. The CV syllables elicited a positive deflection at about 200 ms followed by a negative deflection that peaked around 400 ms. The source models generated placed the dipoles at temporal areas close to auditory cortex for both positive and negative responses. Notably, an additional dipole for the positive peak was localized at the frontal area, at the anterior cingulate cortex (ACC) level. ACC activation has been reported in adults, but has not, to date, been reported in infants during processing of speech-related signals. The frontal ACC activation was earlier but smaller in amplitude than the left and right auditory temporal activations. These results demonstrate that in infancy the ERP generators to CV syllables are localized in cortical areas similar to that reported in adults, but exhibit a notably different temporal course. Specifically, ACC activation in infants significantly precedes auditory temporal activation, whereas in adults ACC activation follows that of temporal cortex. We suggest that these timing differences could be related to current maturational changes, to the ongoing construction of language-specific phonetic maps, and/or to more sensitive attentional switching as a response to speech signals in infancy.